As is well known, exploration for and recovery of oil and gas has long since extended into offshore venues. Early offshore drilling operations were concentrated in relatively shallow waters. However, the number of shallow water drilling sites is finite, while the world's appetite for oil and gas is seemingly unlimited. It has therefore become necessary to conduct offshore drilling operations in waters as deep as 10,000 feet or more.
Offshore drilling operations are frequently conducted from floating platforms known as mobile offshore drilling units (MODUs) with following production operations being conducted using floating production systems. While the mooring in shallow water is relatively straightforward, the successful mooring of MODUs, floating production systems, etc., in deeper water can be problematic.
The traditional method of mooring MODUs, for example, in deeper water involves the use of drag embedment anchors and mooring lines which are stored on the MODU, and which are deployed from the MODU using anchor handling vessels. Some of the latest generation MODUs can carry adequate lengths of wire and chain on board, and are equipped with combination wire/chain mooring winches to moor at maximum depths of 5,000 feet of water. Large anchor handling vessels are capable of deploying and recovering such mooring legs and anchors. In even deeper water, however, the amount of wire and chain that would have to be carried on the MODU becomes too large, and even large anchor handling vessels would have difficulty deploying and recovering such mooring systems in the traditional manner.
Older generation MODUs typically cannot carry enough mooring line to moor in water deeper than about 2,000 to 3,000 feet. This water depth limit can be extended by inserting sections of wire in each mooring leg, or by pre-installing mooring legs prior to arrival of the MODU at location. Both types of extended water depth mooring legs (insert or preset) typically use modern high holding power drag embedment anchors. Large anchor handling vessels are used to install the wire inserts during mooring leg deployment or to pre-install the preset mooring legs.
One drawback to deep water moorings using drag embedment anchors is that such anchors typically cannot handle uplift (vertical load), which requires both that the mooring leg is very long, and that the anchor is set very far from the MODU. In water depths over 6,000 feet the horizontal distance to the anchors can become a problem, since it could be as large as 12,000 feet or 2 nautical miles, and each mooring leg could be as long as 15,000 feet or 2.5 nautical miles. This requires an anchor spread diameter of about 4 nautical miles.
If an anchor system can be used which can handle substantial uplift or vertical load, the anchor radius and mooring line length can be reduced significantly. Driven anchor piles are capable of handling uplift, but cannot be installed in water deeper than about 5,000 feet, nor are they recoverable. For these reasons, driven anchor piles have never been used for deep water moorings.
Mooring systems employing anchors other than conventional drag embedment anchors and driven piles have been proposed heretofore. For example, two types of drag embedded vertically loaded anchors are commercially available. The installation of these drag embedded vertically loaded anchors in deep water requires the connection of a very long length of chain and/or wire between the anchor and the installing vessel in order that a substantially horizontally directed embedment force can be applied to the anchor. Due to its extreme length, the mass of the installing chain and/or wire exceeds that of the anchor by a considerable extent, which causes the anchor to respond to whatever forces may be imposed by the chain and/or wire, including in particular twisting forces. The end result is that it is very difficult to assure the proper orientation, location, and depth of installation of drag embedded vertically loaded anchors installed in deep water.
The foregoing difficulties in installing drag embedded vertically loaded anchors have resulted in renewed interest in the use of suction anchors for deep water installations. U.S. Pat. No. 4,318,641, granted to Hogervorst on Mar. 8, 1992, discloses mooring systems employing suction embedment anchors, which are capable of taking significant uplift or vertical load. One difficulty involved in the use of suction anchors comprises the high cost thereof, which can be $200,000 or more. Another difficulty involves the large size and weight of suction anchors which results in transportation and deployment problems. Therefore, a need exists for an improved method of and apparatus for installing anchors in deep water.
The present invention comprises a method of and apparatus for installing anchors which overcomes the foregoing and other problems long since associated with the prior art. In accordance with the broader aspects of the invention, a plate anchor is temporarily connected to the lower insertion end of a suction follower. A mooring line is connected to the plate anchor and is temporarily connected to the suction follower. The suction follower having the plate anchor secured thereto is lowered from an installation vessel until it engages and partially penetrates the ocean floor under its own weight.
Thereafter, a remotely operated vehicle having a pump mounted thereon is engaged with the suction follower and is utilized to pump water out of the interior of the suction follower. This results in further penetration of the suction follower and the plate anchor secured thereto until the desired depth is reached. The plate anchor and the mooring line are then disengaged from the suction follower, whereupon the operation of the pump on the remotely operated vehicle is reversed. As water is pumped into the suction follower it is forced upwardly out of the ocean floor and is recovered to the installation vehicle. The plate anchor remains embedded in the ocean floor for use in mooring operations, and when a load is applied will orient itself into the correct attitude. The plate anchor may be recovered later if desired.